The purpose of the proposed research is to characterize the mechanism of action of androgens and 5 beta-androstane and 5 beta-pregnane derivatives in erythropoietic tissue (mouse spleen and bone marrow). These studies will provide the background for understanding the pathophysiology of some cases of anemia in man and the basis for a rational design of steroid treatment. The results may also be used to design a compound high in erythropoietic activity but devoid of virilizing side effects. In vivo and in vitro studies will be conducted in normal and androgen insensitive tfm/y mice, animals which lack androgen receptors. The ability of various androgens and 5 beta-steroids to stimulate in vivo erythropoiesis (Fe59 incorporation, reticulocyte count) will be determined and compared with their androgenic activity in mouse kidney and prostate-seminal vesicle. The in vivo and in vitro metabolism of H3-androgens and H3-5beta-steroids will be studied using thin layer chromatography and reverse isotope derivative techniques. The steroid specific distribution and retention of radiometabolites by cytoplasm and nuclei in normal and tfm/y mice will be characterized. The cells responsible for H3-steroid retention will be identified using autoradiography and morphologic identification of individual cells. Assays for quantitating 5alpha- and 5beta-steroid reductase activity will be developed and used to elucidate the hormone control of enzyme activity in erythropoietic tissue. Receptors specific for androgens and 5beta-steroids will be characterized using multiple techniques and compared with those in mouse kidney. The tfm/y mouse will be used to characterize 5beta-receptor uninfluenced by the simultaneous presence of androgen receptors.